At the last "Neuroscience and a Movie," the discussion turned to future programs, and someone mentioned "Neuroscience and Magic." Several of my colleagues are amateur magicians, and one clinician that I co-taught with last year is actually quite a successful magician. I have dabbled in the arcane art myself, and while many would term me a "mechanic" (I prefer tricks that rely on a device or physical prop) I do know and understand how a lot of slight-of-hand and illusion work.
Rather than speak directly about magic tricks, I want to devote a few blogs to the two most common features of magic - illusion and misdirection - and how they each play on features of our brain and nervous system. Today I will discuss illusions (and in particular, optical illusions) and the next blog will cover misdirection. [Today's blog is an expansion on a mailbag question from last year.]
Illusions, and in particular optical illusions, are usually caused by one of two processes. The first is to simply confuse the eyes by playing tricks with what we have come to learn is "normal." For example, in typical 3-D vision, left is closer to the left eye, right is closer to the right eye, close is big, and far away is small. A number of the Escher optical illusions take advantage of violating visual rules and conventions. We can easily follow the lines of the Penrose staircase but the artist violates the rules of perspective by using the same technique of perspective for up/down and near/far. Likewise the Penrose triangle on the same page violates logic, because instead of consistently shading one surface, Lionel and Roger introduce discontinuities that cannot co-exist, thus creating the illusion.
The second method is to tease the eyes by taking advantage of how the retinal ganglion neurons, lateral geniculate nucleus and V1 visual cortex process vision. Figure 1 show the distinction between the real world, and the V1 representation. Because the RGN and LGN are tuned to detect edges, the "fill" in the middle of the text is not represented in V1. That information is not lost, however, color and fill information is transmitted to V2 and V3 second visual areas, which detect shadings, colorations and start to interpret perspective and parallax.
In fact, it has been demonstrated that if a person is shown a Necker Cube-style optical illusion, and told to press a button whenever their perception of the cubes changes from the "top" to the "bottom" view, the pupils dilate briefly. This is just one small way in which the operation of the brain (or – dare I say it – The Mind) can be monitored by a physiological reaction.
One last type of optical illusion that depends on even more complex association of vision and language, and it gets to the types of illusions that magicians employ in their acts: The "Stroop Interference" effect shown in Figure 4 violates the consistency of line vs. shading, but also introduces understanding of the word meaning. This process depends heavily on the multi-sensory association cortices at the intersection of Occipital, Temporal and Parietal lobes – with the added involvement of decision making by the Frontal Lobe. This is one of those phenomena that belies the idea that we only tap a tenth of our brain.
So, how does a magician fool our eyes and our brains? Rather simple, really, the illusionist knows that the brain is looking for fairly simple features, and that combining vision with other information (such as sound, language, or memory) can cause conflicts between what we think we see, and what is really there. We perceive closed lines and circles when they are not, or openings where there are none. We confuse language with color with angle and notice the familiar among the unusual. A large percentage of the magician's craft is also misdirection, and we will discuss that in the next blog.
Until then, don't confuse your brain, take care of it, and watch out for illusions!